Hmm 
We’re gonna need a bigger boat…
I’ve got progressively more assertive with it and it’s still not shifting. The length of 2×2 I was using to get a good swing onto with the mallet sheared before there was even a hint of movement from the head 
Now stripping down the inlet side to see if I can attack it better from that side – but Selina’s just got home and I’ve been ordered out to the pub.
This could be an epic battle …
EDIT 17/08/07 :
Graham ; hitting the head is unlikely to achieve anything except damage to something or someone.
if you have the inlet manifold off insert a hammer handle or similar up an inlet port, and pull the end upwards so effectively you are peeling the head off the block, that will usually break the seal between the head and block without much difficulty